1. Field of the Invention
The present invention generally relates to computer chip substrate manufacture and, more particularly, to reducing the need for ceramic substrates to be reworked after sintering.
2. Background Description
Some ceramic substrates have camber (a degree of non-flatness) that is greater than allowed after sintering. These substrates must be put in a "rework" operation called flattening. In this operation, a mass or load is placed on an alumina interposer, and the load and alumina interposer is placed on the substrates. This assembly of load, alumina interposer and substrates is subsequently "re-fired" at a temperature lower than the sintering temperature.
The flattening operation alleviates the camber problem, but traditionally introduces problems such as adhesion of nonelectrically conducting material onto electrically conducting features of the substrate, in addition to adding cost to the product due to the additional flattening process.
In the past, loads have been used during sintering to keep substrates flat, but with certain substrate materials, such as those containing glass, other problems are generated, such as substrates adhering to the loads. An interface material has been used to separate the load and the substrate, but to make electrical contact, the interface layer must be removed, usually by lapping. Sticking of the load to substrates is also influenced by the amount of load applied to the substrates. To keep a tile flat during the high temperatures usually requires a greater thickness, thereby increasing the minimum mass of the tile and increasing the likelihood of the tiles sticking to a substrate containing glass.
Another problem is that a rigid load will not maintain its flatness over time and will require rework or replacement. Once deformed, it will not make intimate contact with the substrate and not apply the load correctly. Furthermore, a flat, rigid load does not allow for gas flow at lower temperatures which assists binder burnout.
Referring now to the drawings, the problems which have been described are illustrated in FIGS. 1, 2 and 3. In FIG. 1, the sintering process is represented by arrow 11. No load is used to inhibit camber. At first the substrates 12 are flat. As the cycle proceeds, the high heat of sintering makes the substrates curve; i.e., to take a camber. The substrates remain curved when the cycle finishes.
The positive aspect of this free sintering method is that binder burn out is uninhibited. Uninhibited binder burn out ensures that no carbon will be trapped in the substrate after sintering. Carbon burn off occurs early in the sintering cycle. At very high temperatures, carbon will become trapped on the substrate surface, so it is important that carbon burn off occurs before sintering begins.
FIG. 2 shows cross sections of two substrates 22 when they are sintered with a rigid load 23. Again, the cycle is represented by an arrow 21. To begin, the substrates 22 are covered with the load 23. During the high heat portion of the cycle, the load and the substrates remain flat. At the end of the cycle, the load is removed and the flat substrates remain.
While a flat substrate is produced at the end of the cycle illustrated in FIG. 2, the rigid load inhibits binder burn out. If carbon is trapped in the substrate, the substrate will have more porosity and electrical properties will be degraded. In addition, over time this rigid load will become distorted in shape as a result of heating during sintering cycles, requiring reworking or replacing of the load.
A distorted rigid load is shown in FIG. 3. At the beginning of the cycle 31, the distorted load 33 is set on the substrates 32, but does not make intimate contact with the substrates 32. The distortion in this load increases binder burnout because of the lack of intimate contact made by the load. When the high heat sintering begins, the substrates 32 curve (i.e., take a camber) in the heat because the load 33 is not applying pressure evenly to the substrates 32. When the cycle is finished, the load is removed and the substrates 32 retain a camber.
The problem this invention solves is to reduce and/or eliminate the number of times a substrate is required to go through the flattening operation and also to reduce the amount of nonelectrically conducting material adhering as a contaminant to an electrically conducting feature on the substrate.